WASHINGTON (REUTERS) - Republicans desperate to stop Donald Trump from capturing the party's presidential nomination are wrestling with whether to unite behind Mr Ted Cruz, a polarising figure popular with the conservative Tea Party movement.

Mr Cruz, 45, a US senator from Texas, won nominating contests in Kansas and Maine on Saturday, bolstering his argument that he is the leading alternative to Mr Trump, 69, the blunt-spoken billionaire businessman.

Mainstream Republicans are unhappy with Mr Trump's calls to build a wall on the border with Mexico, deport 11 million illegal immigrants and temporarily bar all Muslims from entering the United States.

Many establishment Republicans are reluctant, however, to rally behind Mr Cruz, whom they see as too conservative for the general electorate in the Nov 8 election to succeed Democratic President Barack Obama.

Mr Cruz has run as an outsider bent on shaking up the Republican establishment in Washington. A favourite of evangelicals, he has called for the United States to "carpet bomb" the Islamic State militant group and has pledged to eliminate the tax-collecting Internal Revenue Service and four Cabinet agencies.

But he angered many Republican colleagues when he led the call in 2013 for a stand-off in the US Congress that led to a 16-day shutdown of the federal government.

Republican pollster Neil Newhouse said Mr Cruz had not yet shown an ability to appeal beyond the most conservative voters.

"The way things are going, I think it's extraordinarily unlikely that Senator Cruz becomes the focal point for Republicans who want to stop Trump," said Mr Newhouse, who was lead pollster for 2012 Republican presidential nominee Mitt Romney.

Ms Kim Reem, a member of the executive committee of the National Federation of Republican Women, said both Mr Trump and Mr Cruz were polarising figures within their party.

She said three factions were emerging among Republicans: those supporting Mr Trump, those backing Mr Cruz, and supporters of the party establishment.

"The Cruz folks don't want to yield to supporting Trump and the Trump folks don't want to yield to supporting Cruz, and some establishment folks don't want to support either one of them," said Ms Reem. "I don't see a path to making everybody happy."

Some Republicans argue Mr Cruz is not polling strongly enough in states such as Florida and Ohio. Both will soon hold nominating contests, leading some in the party to question whether backing Mr Cruz would be the best way to stop Mr Trump.

UNFORGIVING MATH

To win the nomination, 1,237 delegates are needed. Mr Cruz has won 300 and Mr Trump 374. US Senator Marco Rubio, 44, of Florida, an establishment favourite still seen by some in the party as an option to Mr Trump, stood to build on his 123 delegates after winning the 23-delegate Puerto Rico primary on Sunday. Ohio Governor John Kasich trails with 35 delegates.

Some establishment Republicans say the best way to stop Mr Trump would be for Mr Rubio to win the 99-delegate Florida contest and Mr Kasich the 66-delegate Ohio primary. Both states award all their delegates to the top vote-getter.

If Mr Cruz, Mr Rubio and Mr Kasich can collectively prevent Mr Trump from getting the needed majority of delegates, they could force a brokered Republican Party convention in July in Cleveland.

But even if Mr Cruz gets the second-highest vote total, he may have trouble claiming the nomination at the convention over Mr Trump.

Former US Senator Trent Lott of Mississippi said he would have a hard time supporting a Cruz nomination. "He'd have to change his tactics and his conduct an awful lot," he said.

Mr Cruz has feuded with party leadership, including Republican Senate Majority Leader Mitch McConnell, and has often accused fellow Republicans of selling out conservative principles.

Although he has been in the Senate for four years, Mr Cruz has not won a single endorsement from any other senator. He touts that on the campaign trail as evidence he is an outsider.

He notably read Dr Seuss' children's book Green Eggs and Ham on the Senate floor as he pushed to repeal Mr Obama's signature healthcare reform law. That politically damaging effort shut down the government for more than two weeks in 2013.

If nothing else, the internal debate reveals a party still deeply divided about how to move forward with Mr Trump and Mr Cruz leading the primary fight.

Mr Slater Bayliss, a Florida Republican who raised money for former Florida Governor Jeb Bush before he dropped out of the race, said: "From my perspective, Senator Cruz's views are indicative of only a very small cohort in our party."

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